1. Field of the Invention
The invention relates to aqueous and/or water-dilutable blocked polyisocyanates, to their preparation and use in optionally self-crosslinking one-component systems.
2. Description of the Related Art
The use of blocking agents for the temporary protection of isocyanate groups has been known for a long time. Blocked polyisocyanates are used for preparing heat-curable 1K PU baking systems which are stable on storage at room temperature. The blocked polyisocyanates therein are mixed, for example, with hydroxyl-containing polyesters, polyacrylates, other polymers, and further constituents of paints and inks such as pigments, cosolvents or additives. Another way to obtain baking varnishes which are stable on storage at room temperature is to block some of the isocyanate groups of polymers containing both blocked isocyanates and hydroxyl groups.
The major compounds used for blocking polyisocyanates are ε-caprolactam, methyl ethyl ketoxime, malonates, secondary amines, and triazole and pyrazole derivatives, as described, for example, in EP-A 0 576 952, EP-A 0 566 953, EP-A 0 159 117, U.S. Pat. No. 4,482,721, WO 97/12924 or EP-A 0 744 423.
Secondary amine blocking agents are described in EP-A 0 096 210. Although the claims of that patent extend to aralkyl-substituted amines as blocking agents, their use is not disclosed in the examples. The use of such amines in aqueous systems is not known from EP-A 0 096 210.
Industrially, the most frequently used blocking agents for isocyanates are ε-caprolactam and butanone oxime. Whereas baking temperatures of around 160° C. are generally employed in the case of ε-caprolactam, blocked 1K baking varnishes for which butanone oxime has been used as the blocking agent can be baked even at temperatures 10–20° C. lower. Admittedly, in some coating systems, the desired level of properties is no longer attained at these baking temperatures. However, even these temperatures are occasionally regarded as being too high, so that there is a need for baking systems which crosslink completely even at lower temperatures.